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Background Spinal anesthesia (SA) is widely used for elective total knee arthroplasty (TKA) but is frequently associated with episodes of low blood pressure. Because baricity influences intrathecal spread, plain bupivacaine (PB) and hyperbaric bupivacaine (HB) may differ in their early hemodynamic effects. This trial compared the early hemodynamic consequences of PB versus HB for SA in elective TKA.Methods This prospective, double-blinded, single-center randomized controlled superiority trial enrolled adult patients undergoing elective TKA under SA, randomized 1:1 to receive 2 mL of 0.5% PB or 2 mL of 0.5% HB. Hemodynamics were measured using automated non-invasive brachial blood pressure every 2.5 min during the first 20 min and every 5 min thereafter up to 1 hour after SA. The primary endpoint was the maximal relative decrease in mean arterial pressure (MAP) from preoperative baseline within 1 hour. Secondary outcomes included nadir MAP, MAP and heart rate trajectories, incidence of intraoperative nausea and vomiting, and ephedrine or atropine use.Results 60 patients were analyzed, 30 in each group. The maximal relative decrease in MAP during the first hour after SA was similar between groups, with mean (SD) decreases of 33.1% (13.3) in the HB group and 33.1% (11.5) in the PB group. Nadir MAP was also similar, with mean (SD) values of 74.2 (9.2) mm Hg in the HB group and 74.1 (12.4) mm Hg in the PB group. Mixed-model analyses showed a significant effect of time on MAP and heart rate, with no significant group effect or group-by-time interaction. Nausea and vomiting occurred in 7 of 30 patients (23.3%) in the HB group and 5 of 30 patients (16.7%) in the PB group.Conclusions In this superiority trial using 10 mg intrathecal bupivacaine for elective TKA, superiority of PB over HB for early hemodynamic stability was not demonstrated.Registration EudraCT 2022-000857-10 and registry URL https://www.clinicaltrialsregister.eu/ctr-search/trial/2022-000857-10/BE.